During the drilling of exploratory wells and the drilling and completing of oil and gas wells, drilling fluid, also called drilling mud, is pumped into the well to maintain a desired pressure within the borehole. Managing the pressure in the well is necessary to inhibit or reduce the influx of formation fluids into the wellbore, while ensuring excessive wellbore pressure does not fracture the formation and lead to significant drilling fluid loss into the formation. Managed Pressure Drilling (MPD) is a drilling process in which the annular pressure profile in the borehole is controlled. MPD helps manage and mitigate potential problems associated with drilling of fractured or karstic carbonate reservoirs, well bore instability, differentially stuck pipe, and drilling formations with a tight margin between formation fracturing pressure and pore pressure.
Choke valves are utilized during managed pressure drilling to control the pressure of drilling mud coming out of the hole, and are a critical part of the drilling process as these control potentially dangerous pressure surges referred to as “kicks.” Failure to adequately control these kicks may lead to a blowout of the well. Choke valves are typically controlled with some form of actuator which may be operated by pneumatic, hydraulic or electric means.
Pneumatic actuated systems have been a primary technology for valve operation for many years. They are simple, low-cost and easy to maintain. By design, pneumatic operation does not create a spark and is often specified for hazardous applications. Compressed air is also readily available in many manufacturing and process control environments.
Pneumatic actuated systems, however, have some limitations. Generally, a valve requires more shift force in a static state than when it is in motion. This trait is commonly referred to as “stick slip.” To overcome stick slip, pneumatic actuated systems build up excess pressure, which can create a rapid movement once the valve is in motion. The resulting overshoot can delay settling on the specified set point or make it difficult to achieve set point at all.
In addition to stick slip, other common problems that can negatively affect pneumatic actuated systems include system demand, air quality, and ambient air temperature. These conditions, in conjunction with operating in an outdoor environment, can further impact system performance due to extreme cold temperatures. Additionally, pneumatic actuated systems typically offer low operating efficiencies on the order of 10 to 30 percent, increasing operating cost.
Hydraulic actuated systems are also expensive to operate due to the continuous power required to maintain operating pressure. However, hydraulic actuated systems continue to be deployed for valve automation processes for a number of reasons: they can be designed as self-contained systems, allowing them to be deployed in remote locations; cost of acquisition can be low to moderate, depending on the size and sophistication of the system; as with pneumatic actuated systems, hydraulic actuated systems do not require significant technical ability to install, configure and deploy; hydraulic cylinders have a very high power density; and the oil used in these systems is nearly incompressible so overall stiffness, positional repeatability and accuracy are improved over pneumatic actuated systems.
At the same time, environmental and system contamination can still negatively affect overall performance in many of the same ways a pneumatic actuated system would be affected. Oil leaking from a system will also diminish performance over time. If the level gets low enough, a leak can create a risk of component damage. Substantial leaks can be considered an environmental hazard. As with pneumatic actuated systems, hydraulic actuated systems also require a high degree of maintenance to ensure proper performance, and performance in extreme ambient air temperatures is reduced.
Electric actuators improve upon many of the shortcomings of pneumatic and hydraulic actuators. However, electric actuators rely on electricity to operate and a loss of electricity to the actuator can create a dangerous situation if the choke valve cannot be manually operated. What is still needed, therefore, is a choke valve actuator which improves upon the shortcomings of prior pneumatic and hydraulic actuators and adequately meets the needs of the industry.